[denton-scratch]

"Defence in depth" is a term with obvious military origins.

You have a relatively thin front-line of defence, with orders to fall back if they are in danger of being overrun. Then you have a very strong second line, manned with assault troops. As the first line falls back, the second line counterattacks.

This strategy was developed by the Germans in WWII, and adopted by the Russians.

I disapprove of it's use in computer security. There, it means something different; it means basically having multiple lines of defence, without any notion of counterattack.